drm/msm/dp: add dp-mst protocol simulator helper

Add dp-mst sideband message protocol simulator to simulate
downstream reply. This is useful for special bridge that
doesn't support sideband message.

Change-Id: I7670abd3505affb9db3232747b86681cea0b4310
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>